In recent years, needs for a display device with a thin flat-panel display (FPD) is being increased, in place of a display device employing a cathode-ray tube which has conventionally and mainly been used. There are various types of FPDs such as a non-self-luminous liquid crystal display (LCD), a self-luminous plasma display panel (PDP), an inorganic electroluminescence (inorganic EL) display, and an organic electroluminescence (organic EL) display.
Among them, the organic EL display has been researched and developed diligently. This is because an element (organic EL element) for display (i) is thin and light in weight and (ii) has further characteristics such as low driving voltage, high luminance, and self-luminous.
The organic EL element includes: a pair of electrodes (anode and cathode) on a substrate; and an organic layer provided between the pair of electrodes. The organic layer includes at least a light emitting layer. The light emitting layer is formed by adding an organic light emitting material to a host material. Generally, the organic EL element further includes (i) a hole injection layer, in which an accepter is added to a host material, between the light emitting layer and the anode and (ii) an electron injection layer, in which a donor is added to a host material, between the light emitting layer and the cathode.
In response to a voltage applied across the anode and the cathode of the organic EL element, (i) holes are injected from the anode into the organic layer and (ii) electrons are injected from the cathode into the organic layer. A hole and an electron, injected from the respective electrodes, recombine with each other in the light emitting layer. This causes an exciton to be generated. The organic EL element emits light by use of light generated when the exciton deactivates.
The light emitting layer is generally made from an organic light emitting material such as a phosphorescent light emitting material or a fluorescent light emitting material. An organic EL element made from the phosphorescent light emitting material has the advantage of high light emitting efficiency and a long emission life. For this reason, recently, the organic EL element in which the light emitting layer is made from the phosphorescent light emitting material has been becoming popular. Further, in order to reduce power consumption of the organic EL elements, there has been developed an organic EL element which is made from a phosphorescent light emitting material having maximum internal quantum yield of 100%.
An organic EL element emitting red light and an organic EL element emitting green light each have employed a light emitting layer made from the phosphorescent light emitting material having a maximum internal quantum yield of maximum 100%. However, an organic EL element emitting blue light has not yet employed a light emitting layer made from the phosphorescent light emitting material having a maximum internal quantum yield of 100%. Instead, a phosphorescent light emitting material having a maximum internal quantum yield of 25% has been employed in the organic EL element emitting blue light.
The organic EL element needs higher energy to emit blue light, as compared with a case where the organic EL elements emit red light or green light. Furthermore, if the energy is obtained from an excited triplet (T1), then it is necessary to confine, within the phosphorescent light emitting material for the light emitting layer, all of T1, electrons, and holes. It is therefore necessary to extremely increase an energy gap between a highest occupied molecular orbital (HOMO) and a lowest occupied molecular orbital (LUMO) not only in a material from which the light emitting layer is made, but also in a material for the periphery of the light emitting layer. However, since the energy gap between the HOMO and the LUMO of the light emitting layer is increased, it is difficult to use, as a host material from which light emitting layer is made, a material in which (i) molecules are conjugated and interacted with each other and (ii) carrier mobility is high. Accordingly, light emitting efficiency of the organic EL element is low, even though it is necessary to apply a high voltage across the anode and the cathode so as to drive an organic EL element which employs a blue phosphorescent light emitting material. This is a problem.
A specific example of a conventional organic EL element 31 whose light emitting layer is made from a blue phosphorescent light emitting material is illustrated in FIG. 8. FIG. 8 is a view illustrating an energy diagram of layers constituting the conventional organic EL element 31 whose light emitting layer is made from the blue phosphorescent light emitting material. In FIG. 8, a hole injection layer 33, a hole transport layer 34, and an electron transport layer 36 employ, as their host materials, NPB (HOMO=5.5 eV, LUMO=2.4 eV), mCP (HOMO=5.9 eV, LUMO=2.4 eV), and 3TPYMB (HOMO=6.8 eV, LUMO=3.3 eV), respectively. A light emitting layer 35 employs FIr6 (HOMO=6.1 eV, LUMO=3.1 eV) as a phosphorescent light emitting material. In a case where holes and electrons are confined within the FIr6, the light emitting layer 35 employs, as a host material, UGH2 (HOMO=7.2 eV, LUMO=2.8 eV) whose energy gap between the HOMO and the LUMO is large. However, the energy gap of the UGH2 is too wide to efficiently propagate the holes from the hole transport layer 34 to the light emitting layer 35. Similarly, the UGH2 cannot efficiently propagate the electrons from an electron transport layer 36 to a light emitting layer 35. Accordingly, even though the organic EL element 31, which employs the blue phosphorescent light emitting material, needs a high driving voltage, its light emitting efficiency is low.
In view of the circumstances, the organic EL element, whose light emitting layer is made from the blue phosphorescent light emitting material, has been devised so as to improve its light emitting efficiency. For example, Non-patent Literature 1 discloses an organic EL element which includes two light emitting layers. Specifically, Non-patent Literature 1 discloses the organic EL element which includes, between a pair of electrodes, an organic layer in which a hole injection layer, a first light-emitting layer, a second light-emitting layer, and an electron injection layer are formed in this order. In Non-patent Literature 1, a hole injection layer and an electron injection layer employ, as their host materials, DTASi (HOMO=5.6 eV, LUMO=2.2 eV) and Bphen (HOMO=6.4 eV, LUMO=3.0 eV), respectively. Further, a first light-emitting layer and a second light-emitting layer employ, as their host materials, 4CzPBP (HOMO=6.0 eV, LUMO=2.5 eV) and PPT (HOMO=6.6 eV, LUMO=2.9 eV), respectively. FIrpic (HOMO=5.8 eV, LUMO=2.9 eV) is added, as a blue phosphorescent light emitting material, to the respective first and second light-emitting layers.
With the arrangement, it is possible to provide an organic EL element in which each of (i) an energy gap between the HOMO and the LUMO of the first light-emitting layer and (ii) an energy gap between the HOMO and the LUMO of the second light-emitting layer, is small. This makes it possible to employ a host material which causes an improvement in each mobility of hole and electron in the light emitting layer. This is because, in an organic deposited film, the hole and the electron are transported by hopping conduction (see Non-patent Literature 2). In order that an electron conducts between molecules by the hopping conduction, it is necessary for wave functions between two electron states, i.e., between a neutral state and a radical anion state to largely overlap each other. Meanwhile, in order that a hole conducts between molecules while hopping, it is necessary for wave functions between two electron states, i.e., between a neutral state and a radial cation state to largely overlap each other. That is, the mobility of the hole and the electron becomes larger as (i) stacking of the neutral state and a radical anion state or (ii) stacking of the neutral state and a radical cation state (π-π interaction) becomes stronger. Further, when the stacking becomes strong, the energy gap between the HOMO and the LUMO becomes small. Accordingly, this arrangement can provide an organic EL element in which (i) a voltage for driving the organic EL element is low, i.e., 4.6V under the condition of 1000 cd/m2 and (ii) a light emitting efficiency is high, i.e., 22 cd/A.